In the field of electric drive vehicles, motor speed control circuits are typically of the chopper or pulse control variety. However, during periods of high speed operation it is advantageous to employ a bypass contactor to shunt the chopper circuit and provide full battery voltage to the electric motor. This control scheme is well known in the art and serves the useful purpose of protecting the chopper circuit from overheating and overcurrent conditions while providing maximum acceleration and velocity.
Typically, these controls monitor accelerator pedal position and energize the bypass contactor after the pedal has been held at its maximum position for a preselected duration of time (e.g. 2 seconds). The delay in operation of the contactor allows the vehicle to smoothly accelerate to a speed relatively near the maximum velocity before directly connecting battery voltage to the motor. Thus, preventing sparking and degradation of the contactor tips and large inrush currents to the drive motor causing jerky operation.
The acceleration rate of electric drive vehicles is dependant upon a number of conditions, including weight of the vehicle, load, current limitations of the chopper circuit, and electric drive motor characteristics. Accordingly, control schemes which force the bypass contactor to be energized after a set period of time cannot optimize acceleration, reduce contactor tip degradation, or prevent large inrush currents to the drive motor. For example, if the delay period is selected to optimize acceleration of an unloaded vehicle, then the delay period will be too short for a loaded vehicle. The contactor will be energized at a relatively low speed resulting in "jerky" operation, sparking, and a large inrush current. Conversely, a delay period that is selected to optimize acceleration of a loaded vehicle will be too long when the vehicle is operated unloaded. The vehicle will quickly accelerate to near maximum velocity and remain at that speed until the time period elapses and the bypass contactor is energized. While the previously mentioned problems are avoided, vehicle performance is less than optimal and the chopper circuit is susceptible to overheating and overcurrent during this extended operation.
The present invention is directed to overcoming one or more of the problems as set forth above.